Internal combustion engines have a lubricant circuit for supplying lubricant to moving components of the internal combustion engine. The lubricant circuit is a circuit which is not completely sealed off from disturbances. Thus, fuel can enter the lubricant via combustion chambers of the internal combustion engine, for example.
The lubricant, generally a lubricating oil, flows through the lubricant circuit over a plurality of operating cycles of the internal combustion engine. During this process, there can be a change in the viscosity of the lubricant. The change in the viscosity has the effect that the lubricant no longer has its original loadbearing capacity, and the corresponding components are thus no longer supplied in the required way.
In addition to the usual aging of the lubricant owing to deposits, abrasion and/or the incorporation of soot, dilution of the lubricant, referred to below as dilution of the lubricating oil, can likewise lead to a change in viscosity. If there is dilution of the lubricating oil, this means that there are proportions of fuel in the lubricant which result in the change in viscosity of the lubricant. The viscosity of the oil is significant for the lubricating effect of the oil. If the viscosity of the oil falls too much, the required lubrication of the internal combustion engine can no longer be reliably guaranteed.
Dilution of the lubricant can have various causes, e.g. frequent cold starts and short journeys, insufficient mixture preparation, insufficient warm-up phases combined with start-stop operation or electric operation in the case of hybrid vehicles.
German Laid-Open Application DE 10 2013 216 215 A1 discloses a method for reducing dilution of the lubricating oil, wherein, in particular, the internal combustion engine is operated for a certain time in a mode in which an engine temperature exceeds a certain threshold temperature.
DE 60 2005 000 084 T2 discloses a control method for an exhaust gas purification system of an internal combustion engine which brings about regeneration of a diesel particulate filter while taking account of possible dilution of the oil. The regeneration of the diesel particulate filter is carried out substantially in the steady-state or stationary condition of a motor vehicle having the internal combustion engine.
German Laid-Open Application DE 10 2012 112 794 A1 discloses a method for an internal combustion engine for determining a proportion of fuel in the oil. If a certain proportion of fuel is detected in the lubricating oil, there is the possibility of providing a remedy in various ways that are proposed. This remedy can be to change the oil, for example. In the case of a hybrid drive for a motor vehicle, i.e. in the case of a combination of an electric motor with the internal combustion engine, it is possible not to switch off the internal combustion engine for a certain period of time. With the aid of the method proposed, the proportion of fuel in the lubricating oil can be determined The internal combustion engine is controlled in accordance with the proportion of fuel determined, wherein, apart from the possibilities already mentioned, changing the operating point by shifting the load point of the internal combustion engine to increase a coolant and/or oil temperature is also mentioned.